1. Field of the Invention
The present invention relates to a high frequency switch, and particularly to a high frequency switch used for switching of an RF signal used for a radar and a communication device operated in a millimeter wave band.
2. Description of the Related Art
The application of a microwave device has been increasingly advanced with the progression of computerization. A microwave device of a cellular phone using a relatively low frequency, a microwave device used in information communication by a high frequency much higher than the low frequency of the cellular phone, a microwave device used in communication apparatuses such as a radar for automobile use, an observation satellite, etc. each of which uses a high frequency much higher than the above frequency in information communication, etc. have been applied in many fields.
Of these, the radar using the millimeter wave-band microwave device is used to recognize a forward vehicle or automobile and constitutes part of a system for performing automobile's collision avoidance.
A system for scanning nine directions has been used in a millimeter wave-band radar for detecting a forward automobile to carry out automobile's collision avoidance. This is used assuming that an automobile runs along three lanes. This system needs to perform the scanning of nine directions with a view toward scanning three directions for each lane. Therefore, there is a need to provide a nine-channel switch which switches an RF signal over to the nine directions.
As a high-speed switch, a small-sized and high-speed high frequency switch using a microwave IC (MIC) has been realized in recent years. Further, it is configured as an MMIC (Monolithic Microwave IC) in which connecting lines and switching elements are integrally formed on a semiconductor substrate, thereby enabling further speeding-up and miniaturization of the switch.
When the nine-channel high frequency switch is configured, the most basic configuration is proposed wherein nine SPST (Single Pole Single Throw) switches formed on a GaAs substrate are used. That is, a signal line that branches into nine branch lines from one input terminal via a branch point is formed on a dielectric substrate, and SPST switches are respectively provided in the individual branch lines of the signal line to thereby configure a nine-channel switch.
Alternatively, a nine-channel switch is configured wherein four SP3T (Single Pole 3 Throw) switches formed on a GaAs substrate are provided on a dielectric substrate, an input end of the first SP3T switch is connected to input signal lines on the dielectric substrate, and input terminals of the three other SP3T switches are respectively connected to three output terminals of the first SP3T switch to thereby set nine output ends.
As a known example of a conventional high frequency switch, there has been disclosed a configuration wherein by adopting a switch circuit using a distributed constant FET for SPDT, a less passage loss can be obtained in the case of switch ON and high isolation can be expected in the case of switch OFF (see, for example, Japanese Patent Laid-open No. 2002-33602, paragraph numbers [0013] and [0014] and FIG. 1).
Further, as a known example of another high frequency switch, there has been disclosed a high frequency switch which includes a plurality of tristate switches which are connected in tournament form by strip lines and wherein the lengths of strip lines from branch points of adjacent lines connected to the individual switches to their corresponding switches are so adjusted that real parts of impedance obtained by viewing the turned-off switches from the branch points of the adjacent lines connected to the individual switches become maximum and imaginary parts thereof reach 0, and the lengths thereof from branch points of roots of the adjacent lines connected to the respective branch points to their corresponding branch points are adjusted to integral multiples of a ½ wavelength (see, for example, Japanese Patent Laid-open No. 2000-261218, paragraph number [0006] and FIG. 1).
Furthermore, as a known example of a further high frequency switch, there has been disclosed one wherein when four or more receiving antennas are switched in a holographic radar, single pole 2 throw (SPDT) or single pole 3 throw (SP3T) unit switches, e.g., planar circuit type high frequency switches like an MMIC and an HIC are utilized and combined in the form of a tournament to thereby realize multi-switching (see, for example, Japanese Patent Laid-open No.2000-155171, paragraph number [0005] and FIG. 5).
Still further, as a known example of yet another high frequency switch, there has been disclosed an example wherein one transmission line is provided in the direction of the input of a signal and the remaining two transmission lines are provided in the directions of 90°, 180° and 270° with respect to the input direction of the signal to make equal distances to respective output terminals, thereby forming 3-distributed switches low in loss and equal in loss (see, for example, Japanese Patent Laid-open No. 2000-294568, paragraph number [0031] and FIG. 5).
However, the nine channel switches respectively having such a configuration that the nine SPST switches are used and such a configuration that the four SP3T switches are used, are hard to achieve a size reduction because the dielectric substrates become large. Further, since the characteristics of the respective SPST and SP3T switches to be used are hard to match and the number of parts increases, the electrical characteristics of individual channels are apt to vary due to variations in bonding wire at their packaging. It was also difficult to constitute the high frequency switch having the nine channels uniform in electrical characteristic. It is thus considered that in order to constitute the nine-channel switch uniform in electrical characteristic, the MMIC configured by the one-chip GaAs substrate constitutes switches that branch into nine.
Isolation is considered as one of major parameters indicative of switch performance of a high speed switch. Since, however, the branch lines can be laid out at intervals of 90° with respect to the input signal line in the SP3T switch, it was easy to ensure the isolation between the respective branch lines. However, when the nine-branched switches are constituted by the MMIC simply configured of the one-chip GaAs substrate to configure the high frequency switch having the nine channels uniform in electrical characteristic, the branch angle formed between the respective branch lines becomes an acute angle. Therefore, a problem arose in that isolation between respective terminals for the nine channels was hard to obtain.